imx: kontron-sl-mx8mm: Add support for loading from SPI NOR in SPL

This was initially enabled, but got accidentally dropped while
migrating Kconfig options and resyncing the defconfig in:

commit 9802154a ("configs: Resync with savedefconfig")

Let's enable this again to be able to boot from SPI NOR.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>